The Hornsby Plateau is a dissected sandstone

plateau In geology and physical geography, a plateau (; ; ), also called a high plain or a tableland, is an area of a highland consisting of flat terrain that is raised sharply above the surrounding area on at least one side. Often one or more sides ...

lying to the north of

Sydney Harbour Port Jackson, consisting of the waters of Sydney Harbour, Middle Harbour, North Harbour and the Lane Cove River, Lane Cove and Parramatta River, Parramatta Rivers, is the ria or harbor, natural harbour of Sydney, New South Wales, Australia. T ...

that rises 200 metres. The plateau is a part of the larger

Sydney Basin The Sydney Basin is an interim Australian bioregion and is both a structural entity and a depositional area, now preserved on the east coast of New South Wales, Australia and with some of its eastern side now subsided beneath the Tasman Sea ...

structure.

Geography

The North Shore and the

Berowra Valley National Park The Berowra Valley National Park is a protected national park that is located in northern Sydney, New South Wales, Australia. The national park is situated approximately north-west of the Sydney central business district. Located within ...

are located on the Hornsby Plateau. It is separated from the Blue Mountains Plateau by the passage of the Colo River. The plateau rises from the

Cumberland Plain The Cumberland Plain, an IBRA biogeographic region, is a relatively flat region lying to the west of Sydney CBD in New South Wales, Australia. Cumberland Basin is the preferred physiographic and geological term for the low-lying plain of the P ...

in the south along a warp line increasing from the proximity of

Cattai Cattai is an historic suburb of Sydney, in the state of New South Wales, Australia. It is located 44 kilometres north-west of the Sydney central business district and 30 kilometres north-west of Parramatta. It is in the local government ...

Botany Bay Botany Bay ( Dharawal: ''Kamay''), an open oceanic embayment, is located in Sydney, New South Wales, Australia, south of the Sydney central business district. Its source is the confluence of the Georges River at Taren Point and the Cook ...

. The plateau's elevation ranges between 200-220 metres, with gullies that fall 80-100 metres deep along drainage lines. Steep slopes with drastic ascension of over 100 metres are present. The soil type is generally lithosols, which has low fertility and is normally less than 500 mm deep.

Geology

Rock deposits in the

Sydney Sydney ( ) is the capital city of the state of New South Wales, and the most populous city in both Australia and Oceania. Located on Australia's east coast, the metropolis surrounds Sydney Harbour and extends about towards the Blue Mounta ...

area were

tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

freshwater sediments, which were a rough

gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...

ly layer up to 8 metres thick located beneath

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel ...

and sand at a mean depth of 6 metres. As time went by,

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movement ...

transformed much of the sandy silt into

siliceous Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

clay. At the end of the Tertiary period, or about 80 million years ago, earth movements created an upheaval of 600 metres in most of coastal eastern Australia, due to the rifting and opening of the

Tasman Sea The Tasman Sea (Māori: ''Te Tai-o-Rēhua'', ) is a marginal sea of the South Pacific Ocean, situated between Australia and New Zealand. It measures about across and about from north to south. The sea was named after the Dutch explorer ...

. Thus, these movements created the Blue Mountains and Hornsby Plateaux, with the lagging behind of the peneplain becoming the Cumberland Plain. The plateau was scoured by

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...

streams, which perforated a labyrinth of V-shaped

valley A valley is an elongated low area often running between hills or mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams ove ...

s with intruding rocky ridges into the Hawkesbury Sandstone. The drainage activity is ascertained by the Hawkesbury Sandstone's impotence, which were employed by early watercourses. As the creeks carved deeper into the plateau, they began undercutting the sandstone walls of the valley, and as such, blocks of sandstone fell down, thus creating cliff lines.Herbert, C. & Helby, R. (eds) 1980, A Guide to the Sydney Basin, Dept. of Mineral Resources, Geological Survey of NSW, bulletin no. 26. Due to its hilly topography, the plateau was slow to develop. It has two main sedimentary rock layers:

Hawkesbury sandstone Sydney sandstone is the common name for Sydney Basin Hawkesbury Sandstone, one variety of which is historically known as Yellowblock, and also as "yellow gold" a sedimentary rock named after the Hawkesbury River north of Sydney, where this ...

, consisting mostly of sandstone, but with some

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...

, and the overlying Ashfield Shale, which supports richer vegetation. There are also a few volcanic intrusions throughout the plateau.

References

{{Coord, -33.70490, 151.09901, format=dms, type:city_region:AU-NSW, display=title Landforms of New South Wales Plateaus of Australia Geography of Australia Hornsby, New South Wales Hornsby Shire Geography of Sydney Landforms of Sydney

Geography

Geology

See also

References